Absorption chillers are heat driven refrigeration machines which have been manufactured for several decades. Prior to the mid 1970's "energy crisis" when natural gas was relatively inexpensive, simple absorption machines operating at relatively low efficiency, typically 0.5 to 0.7 coefficient of performance (COP), were economically attractive. Those machines were characterized as quiet, vibration-free, reliable machines whose initial cost per ton of capacity was somewhat higher than equivalent electric equipment. With the increase in natural gas prices since the mid 1970's however, conventional absorption chillers have lost their economic attractiveness.
In absorption refrigeration cycles, a secondary fluid (the absorbent) absorbs a primary fluid (gaseous refrigerant) that has been vaporized in an evaporator. In a typical single-effect absorption refrigeration system, water is used as the refrigerant and lithium bromide as the absorbent. The refrigerant/absorbent combination is known as the solution pair. Other chemical combinations (solutions) have been used, or have the potential for use, in absorption cycles.
The mode of operation for a single-effect absorption chiller is well known in the art. Refrigerant vapor is produced in the evaporator at a temperature somewhat below that of the heat load. The refrigerant vapor is exothermically absorbed by a concentrated absorbent solution entering the absorber. The heat of absorption is then transferred to a heat sink, such as cooling water, at the absorber. The now dilute absorbent solution is pumped to the generator, where it is concentrated again and returned to the absorber. External heat is supplied to the generator to supply the energy required to separate the refrigerant from the absorbent. The refrigerant is condensed at the condenser and is returned to the evaporator while the concentrated absorbent is returned to the absorber. A concentrated absorbent is returned to the absorber. A heat exchanger between the absorber and generator is also part of the system, exchanging heat to the dilute absorbent from the concentrated absorbent solution.
The above process takes place between two pressures: a lower pressure prevailing in the evaporator-absorber section and a higher pressure in the generator-condenser section. The operating temperature limits of the refrigerant/absorbent combination (solution pair), are determined by the chemical and physical properties of the pair.
The cooling thermal efficiency (COP) of a single-effect cycle is typically about 0.5 to 0.7. Modifications of the basic cycle do not bring the coefficient of performance over a threshold of unity, e.g., heat required to generate one pound of refrigerant is not less than the heat taken up when this pound evaporates in the evaporator. Performance can be improved by using the double-effect evaporation principle practiced by the chemical industry for decades and a double-effect generator. With a water lithium bromide pair, two generators can be used. One, at high temperature and pressure, is heated by an external source of thermal energy. A second, at lower pressure and temperature, is heated by condensation of the vapor from the first generator. Condensate from both generators moves to the evaporator. This enables the external thermal energy to be effectively utilized twice in the high and low temperature generators, thereby increasing the overall thermal efficiency as compared to single-effect absorption systems. The thermal efficiency of double-effect cycles is typically about 1.0 to 1.2 with one double-effect absorption machine reported to have attained at 1.3 COP.
Dual loop absorption cycles have been proposed and are being developed in which two separate absorption loops, a high temperature loop and a lower temperature loop, are combined to offer desirable features beyond those attainable with double-effect systems. One previous dual loop system is shown in U.S. Pat. No. 3,483,710 (Bearint) and features a high temperature condenser in heat exchange relation with a low temperature generator. Another system, described in U.S. Pat. No. 4,542,628 (Sarkisian et al), has a high temperature condenser and a high temperature absorber in heat exchange relation with a low temperature generator with simultaneous heat exchange between the high temperature evaporator and the low temperature condenser and/or low temperature absorber.
Additionally, the latter dual loop thermodynamic cycle (but not a machine concept) has been separately proposed by other absorption researchers, viz., P.D. Iedema, The Absorption Heat Pump with Lithium Bromide/Zinc Bromide/Methanol, WTHD No. 162, Laboratory of Refrigeration and Indoor Technology, Department of Mechanical Engineering, Delft University of Technology, The Netherlands, April 1984. In these prior dual loop heat pump concepts, the dual loop absorption cycle thermal efficiency is approximately the same as double-effect machines for air conditioning and refrigeration applications, since the external thermal energy is effectively utilized twice to produce the desired cooling effect in the evaporator.
In U.S. Pat. No. 4,531,374 (Alefeld), more than one hundred theoretical multi-stage absorption cycles or multi-stage absorption/compression cycles are disclosed. However, the disclosed cycles extend only to the equivalent of a five-effect absorption cycle system.
While relatively high efficiency double-effect chillers have been developed and manufactured with a COP of 1.0-1.3, even at these higher efficiencies such devices are still only marginally economic in the U.S. Recent innovations have shown improved performance, such as those described in the above-identified Sarkisian et al patent and in U.S. patent application Ser. No. 933,943 filed on Nov. 24, 1986. However, there still is worldwide interest in improving absorption/refrigeration cycles as well as developing new cycles.